The present invention relates to an apparatus for thermal diffusion of semiconductor devices so that semiconductor devices with uniform quality may be mass produced.
It has been well known in the art that in the process for fabrication of semiconductor devices such as diodes, transistors and the like by doping various impurity elements into substrates of a silicon semiconductor or the like, thermal diffusion methods have been widely used to diffuse the impurities into the substrates (hereafter merely denote as wafers). The prior art apparatus for carrying out the diffusion process includes a diffusion furnace such as an electric furnace which is controlled with a higher degree of accuracy to minimize temperature variation in the diffusion furnace. In general, the prior art diffusion furnace includes a furnace tube made of quartz of the like and extended through the diffusion furnace, and a plurality of wafers to be processed are placed in a predetermined position within the furnace tube and kept for a required length of time at a predetermined temperature for diffusion. More particularly wafer boats each made of a refractory material such as quartz or graphite and adapted to carry a predetermined number of wafers to be processed are arrayed and inserted into a furnace tube to be placed in a predetermined position, and after being kept for a predetermined diffusion time, the boats are withdrawn from the furnace tube when the thermal diffusion of the wafers is completed. Depending upon the required extent of diffusion, a suitable gas such as nitrogen or oxygen gas is introduced into the furnace tube from one end thereof to maintain a required high temperature atmosphere.
The electrical characteristics of semiconductor devices such as diodes, transistors or the like fabricated by the diffusion of impurity elements into semiconductor wafers are, in general, much influenced by the concentration and the depth of penetration of impurities into wafers which in turn are dependent upon the temperature of the diffusion atmosphere in which the wafers are placed. Therefore unless the diffusion temperature is controlled with a higher degree of accuracy, the required electrical characteristics of semiconductor devices cannot be obtained. In general, in the thermal diffusion apparatus used in the process for fabricating semiconductor devices, the temperature variation in a furnace tube of a diffusion tube must be controlled within .+-. 0.5.degree. C of a setting temperature. Meanwhile when the number of semiconductor wafers to be inserted into a furnace tube is increased, the length of a controlled space must be increased, accordingly.
However, when the construction of a heater and a control system are taken into account a volume or space which may be effectively used in practice for diffusion of wafers, the increase in length of a diffusion zone is not economical, resulting in large size and high construction and operation costs of a diffusion apparatus. Consequently, the manufacturing costs of a semiconductor device are inevitably increased.
The electrical characteristics of semiconductor devices fabricated by thermal diffusion are greatly dependent not only on the temperature of the atmosphere, the concentration and depth of penetration of impurities but also on the crystal structures of wafers to be processed. For instance, when the concentration and penetration depth are the same but the speed with which wafers are inserted into and withdrawn out of a high temperature furnace tube or diffusion zone is different, thermal shock exerted to each wafer may differ, thus resulting in a difference in crystal structure. Consequently the electrical characteristics are different from each other. In the fabrication of well known planar semiconductor devices, the residual stress distribution at the interface between an oxide film and a semiconductor crystal varies depending upon the speed at which a wafer is inserted into and withdrawn out of high temperature diffusion zone. As a result, the variation in electrical characteristics occur with the resultant considerable decrease in yield rate.
As described above, according to the prior art thermal diffusion processes and apparatus, not only the lot variation; that is, the variation in electrical characteristics among the wafers in each lot or batch due to the unstable temperature and atmosphere distribution or gradient within the furnace tube or diffusion zone but the variation in electrical characteristics among the lots or batches due to the difference in operating conditions including the condition under which each batch is inserted into and withdrawn out of the furnace tube or diffusion zone, cannot be eliminated or minimized. To overcome these problems many attempts and efforts have been made, but so far no satisfactory means has been proposed yet.
In general, the thermal diffusion process comprises a cleaning step, a drying step and a diffusion step, and apparatus for automatically carrying out the cleaning and drying steps have quite recently been introduced, but the diffusion step still remains a batch operation. The full automation of the thermal diffusion process cannot be attained without the automation of the diffusion step, and consequently the large-scale mass production of semiconductor devices cannot be attained.
In view of the above, one of the objects of the present invention is to provide a process and apparatus for thermal diffusion of semiconductor wafers, wherein a plurality of wafer boats each made of a refractory material and adapted to carry a predetermined number of wafers to be processed are continuously transported in line through a high temperature, thermal diffusion zone or a furnace tube so that each wafer may experience substantially the same thermal history and consequently wafers with a minimum variation in characteristics may be mass produced on a large scale .